The use of metal bumps to bond external metal beam leads to contact patterns on semiconductor chips is well known in the art Beam lead bonding, flip-chip bonding, and tape automated bonding (TAB), for example, comprise examples of semiconductor chip mounting processes that utilize metal bumps formed over selected regions of the wiring pattern on a chip. In thermal compression bonding, a combination of heat and physical pressure is used to bond metal bumps on the chip to external metal connections. Those external connections can be provided, for example, on an external chip or a flexible tape, and positioned so as to be in facing relationship to the metal bumps.
Referring to FIG. 1 a prior art bump/chip structure is shown which has been found to present potential corrosion problems leading to chip malfunction. A semiconductor substrate 10 has formed thereon a conductive contact land 12 to which a metal bump 14 is affixed.
A glassy, passivating layer 16 overlays the surface of substrate 10. During the processing of substrate 10, an opening is etched in layer 16 by placing a mask on its upper surface, followed by an etch. The etched opening has generally been made somewhat larger in cross section than bump 14 in order to assure that the bump, when it is deposited, is seated squarely on contact land 12.
The composition of bump 14 includes a bonding layer 20 of chromium on which an aluminum pedestal 22 was deposited. Next, an additional layer of chromium 24 is laid down upon the uppermost surface of aluminum pedestal 22, followed by layers of copper 26 and gold 28.
As a consequence of this structure exposed collar areas 18 were created where corrosion could attack the uppermost surface of contact land 12. While attempts have been made to passivate collar region 18 through the application of polymeric overcoats, the results have not been totally satisfactory.
Other prior art showing methods for forming bumps on conductive lands on semiconductors can be found in U.S. Pat. Nos. 4,042,954, 4,427,715, and 3,874,072. In U.S. Pat. No. 4,042,954 to Harris there is shown a method for forming metal bumps which utilizes a multilayer transition structure of Cr, AlCr, Cr and Au. This transition structure is used to connect a nickel-under-copper bump to an aluminum metal pattern on a semiconductor chip. The aluminum metal pattern contacts selected regions on the chip through an SiO.sub.2 passivating layer.
In U.S. Pat. No. 4,427,715 to Harris, another method is shown for forming metal bumps wherein a bump is centered over a pad so as to cover a window in an intermediate passivating layer. The positioning and size of the bump is selected relative to the pads such that during thermal compression bonding, the periphery of the bump does not extend over the periphery of the pad. This arrangement is purported to prevent failures caused by cracking in the passivating layer.
In U.S. Pat. No. 3,874,072 to Rose et al., a method is shown for forming mushroom-shaped metal bumps which incorporates multiple layers of varying metals. Briefly, a nickel mushroom cap is bonded to an aluminum layer through intervening nickel, chromium layers respectively. The aluminum layer is deposited on an aluminum pad through a window in a thin glass passivating layer. Thin layers of gold tin and gold are formed sequentially over the nickel cap.
The above patents suffer from a number of disadvantages. Some do not anticipate the corrosion problems which may occur due to either a lack of a good bond between an aluminum bump and an underlying insulating structure or from exposed contact land metallurgy. Others do not anticipate that the misalignment of a bump can cause cracking of an underlying passivating layer.
In summary, there is a need in the art for a metal bump process, useful in thermal compression bonding which is both inexpensive to manufacture, inhibits corrosion due to exposed metallurgy, and maintains optimum sub-bump structures.
Accordingly, it is an object of this invention to provide a new and improved metal bump for use in thermal compression bonding processes on semiconductor chips.
It is another object of this invention to provide an improved method for manufacturing interconnecting metal bump structures wherein passivation layer cracking is avoided.
It is still another object of this invention to provide a metal bump structure that does not leave regions which expose underlying contact metallurgy which is corrosion-sensitive.
It is yet another object of this invention is to provide a metal bump/semiconductor structure wherein a glassy passivating layer may be employed.